AR-7859 Int UG/PG (BSc Hons) II Sem-2013 Subject: BIOTECHNOLOGY Paper-II (GENETICS)

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1 Answer to Qn 1 AR-7859 Int UG/PG (BSc Hons) II Sem-2013 Subject: BIOTECHNOLOGY Paper-II (GENETICS) 1. (d) 1/16 2. (b)!:2:1 3. (a) Variation 4. (b) 25 cm 5. (d) ds circular DNA 6. (a) Yeast 7. (b) Three 8. (a) Triticale 9. (b) Father is hemophilic and Mother is carrier 10. (c) Downs syndrome Answer to Qn. 2 (Subjective Type) 1. Explain the following terms with the help of suitable examples: (a) Incomplete dominance: When a hybrid obtained by crossing two pure parents showing contrasting characters produce a new phenotype different from the dominant parent, the phenomenon is called incomplete dominance.it is an example of deviation from normal Mendel s Law of Dominance which is based on the phenotypic ratio of 3:1 and genotypic ratio of 1:2:1. e.g. Flower Colour in Mirabilis jalapa When a Red flowered Plant is crossed with a white flowered plant, the F1 hybrids are always pink flowered. This is due to the fact that none of othe two alleles (R or r ) are completely dominant as described below: Red White RR X rr F1 Hybrid= (PINK) = A new character Initially, it was considered to be a case of blending inheritance. However, the result of selfing of Pink flowered produced 25% each of pure red and

2 pure white flowred plants suggesting thereby that the alleles do not blend, rather their products (pigments) get blended to produce a new colour (pink). On selfing pink flowered Plants Pink X Pink Gametes R r R RR Red Pink r Pink White Phenotypic and Genotypic Ratio (same) =1:2:1 (b) Co-dominance: This is also an example of deviation from normal Mendelian ratio of monohybrid cross. The phenomenon may be explained same as above (incomplete dominance), the only difference is that the F1 hybrid in case of co-dominance always produces an intermediate character due to simultaneous expression of both the alleles (dominant and recessive), whereas in case of incomplete dominance, the F1 hybrid shows a new character due to blending of the products of two alleles. e.g. Coat (skin) color in calf, AB Blood group in human Red cow White bull RR X rr F1 Hybrid= (SPOTTED) = An intermediate character On inbreeding two spotted animals Spotted X Pink Gametes R r R RR Red Spotted r Spotted White In case of co-dominance also the same Phenotypic and Genotypic Ratio of 1:2:1 is obtained. (c) Lethal genes: Certain genes (dominant or recessive) when present in homozygous condition, produce lethal or toxic effect leading to death of either dominant homozygous progeny (called Dominant Lethal ) or recessive homozygous progeny (called Recessive Lethal ). (i) Dominant lethal : e.g. Mice which contains a dominant gene L for lethality but also contains a recessive allele l which produces antidote of lethal factor. As such the hybrids (Ll) do survive but dominant homozygous progeny (LL) dies out leading to a deviation from the normal monohybrid phenotypic ratio of 3:1.

3 Ll X Ll LL Ll Ll Normal ratio: Dies Survive Survive (Observed Ratio: 2:1) (ii) Recessive Lethal : e.g. Sickle cell anemia in human, which is caused due to recessive homozygous genes present on autosomes. The disease develops due to a gene mutation in the β-chain of normal Hb A in which the 6 th amino acid glutamic acid is replaced by valine leading to loss of oxygen biding ability of Hb causing RBC to assume a sickle shape. The mutant gene is denoted by Hb S. If the two parents are carrier (heterozygous), 25% of recessive homozous progeny show lethality leading to deviation from normal phenotypic ratio of 3:1. Hb A /Hb S X Hb A /Hb S Hb A /Hb A Hb A /Hb S Hb S /Hb S Normal ratio: Survives Survives Dies (Observed Ratio: 1:2) (d) Pleiotropism : When a single gene controls the expression of two or more characters, the gene is called pleiotropic gene and the phenomenon is called pleiotropism. e.g. Flower colour and colour of the seed (cotyledons) in Sweet pea (Lathyrus odoratus) is controlled by a single gene. (e) Complementary genes : Two unlinked dominant genes controlling the expression of the same character are brought together through hybridization, they show interaction leading to production of new phenotypes thus deviating the normal dihybrid phenotypic ratio from 9:3:3:1. Complementary genes are one of the best examples of interaction of genes. e.g. Flower colour in sweet pea (Lathyrus odoratus) is controlled by two dominant genes C and P. None of the two dominant genes individually produce any pigment. As such, the flowers are white when only one dominant gene is present. However, if two pure white flowered plants are crossed together, the ratio deviates from 9:3:3:1 to 9 (Purple):7 (White). White Flower White Flower CCpp X ccpp F1 Hybrid: CcPp Purple Flower

4 On Selfing Purple Flower (F1 hybrid) plants: Purple Flower Purple Flower CcPp X CcPp Expected Ratio: CP Cp cp cp Complementary Gene Ratio: Observed: (9Purple) ( 7White ) 9:7 The biochemical explanation of complementation is that none of the two dominant genes (either C or P) can synthesize the pigments alone due to block at a particular step of metabolic pathway leading to pigment production. However, when both come together in a dihybrid (CcPp), the pathway is re-constituted producing pigments and purple colour of the flower. Q2. Describe the inheritance of skin colour in human considering that skin are colour is controlled by two dominant genes, giving number of phenotypic classes and phenotypic ratio. The skin colour in mammals, including human being, is due to production of a pigment, melanin, the production of which is under the control of many dominant genes (called polygenic inheritance). Such characters show a gradation pattern from maximum (100%) melanin producer (called Negro) to minimum (0%) melanin producers (called albino). The quantity of melanin produced is directly proportional to the number of dominant genes present in a human, because recessive genes do not produce melanin. As mentioned in the question, if the skin colour is controlled by two dominant genes, A and B, the pattern of inheritance of such characters should follow the normal Menelian pattern as observed with dihybrid cross. However, the results show a great deal of deviation from the normal rule due to its dependence on the quantity of gene products, as explained by the following mating experiments: Negroes X Albinos AABB aabb Melanin=(4 Dom. genes x 25%= 100%) (4 Rec. genes x 0% =0%) F1 Hybrid = AaBb Melanin=(2 Dom. genes x 25%= 50%) Intermediate Skin color (Called Mulattos)

5 The inbreeding of mulattos is likely to produce 16 progeny. However, the deviation observed from Mendelian ratio is as follows: Phenotypic Class= 5 (instead of 4 as in dihybrid cross) Phenotypic Ratio= 1/16 : 4/16 : 6/16 : 4/16 : 1/16 Dom (D)/Rec (R)= 4D/0R 3D/0R 2D/2R 1D/3R 0D/4R Melanin (%) = Skin color = Negro Less than Mulattos Less than Albino Negro Multtos 3. Explain the following: (a) Evidence of cytoplasmic factors The cytoplasm contains DNA in both chloroplast and mitochondria, the nature of which is ds circular DNA. As such some of the characters are controlled by these cytoplasmic factors. The evidence for the existence cytoplasmic factor was given by Darlington (1940) who coined the term Plasmagenes. The inheritance of characters borne on such factors are called cytoplasmic inheritance or extra-nuclear inheritance. (b) Maternal influence The phenomenon was first observed by Carl Correns while studying the inheritance of plastid colors in Mirabilis jalapa. In this plant, the leaves are found in three colours: Green (G), Yellow (Y) and Spotted or Variegated (V). Correns performed crosses in pair-wise combinations and observed that the results did not obey Mendel s concept of reciprocal crosses give the same results. Correns explained the phenomenon as follows: Fig.: Correns Experiment on Plastid inheritance The body of any organism develops from a fertilized egg. The size of egg is several times more than those of male gametes. As such the egg contains more cytoplasm and cytoplasmic organelles like chloroplasts and mitochondria. The male gamete, on the other hand, has little or no cytoplasm and as such no organelles. During formation of zygote, only the female gamete contributes more cytoplasm containing organelles and more plasmagenes. With respect to nuclear genes, both male and female gametes have equal contribution in the zygote formation. The nuclear genes coming from the two parents show dominant-recessive relationship and Mendelian pattern of inheritance. Due to this the progeny shows more characters of the female parent due to the genes present in choloroplast (plastids) appear in progeny. Since such genes

6 do not show dominant-recessive relationship, being always in haploid state, the reciprocal crosses do not produce the same results for plastid colour. The phenomenon in which the female has more influence on the phenotype of an organism is termed as Maternal Influence. 4. Explain the phenomenon of meiotic non-disjunction? How does it contribute to the development of syndromes in human? The failure of separation of homologous chromosomes of a bivalent during Anaphase I of meiosis leading to formation of two types of gametes, one having (n+1) and the other having (n-1) number of chromosomes is known as meiotic non-disjunction. Fig.: Meiotic Non-disjunction leading to abnormal gametes In normal meiosis, pairing of homologous chromosome called synapsis (junction) leading to formation of bivalents during zygotene substage of Prophase I and separation of homologous chromosomes (disjunction) during Anaphase I are the two natural phenomena. Abnormalities may arise when the paired homologues do not separate (non-disjunction) due to mal-functioning of spindle fibers. Under such a situation the gametes produced after completion of meiotic divisions do not contain equal number of chromosomes due to abnormalities occurred in Anaphase I. Since the gametes produced after non-disjuction have unequal chromosome numbers, their fusion with even normal gamete from the other mating partner (male or female) leads to production children having either less chromosome number than the normal human, i.e 2n=45 (AA+XO as in case of Turners syndrome) or more chromosome number, i.e 2n=47 (AA+XXY as in case of Klinefelter s syndrome or AA+21 autosomes in case of Downs syndrome. The probability of children showing irreversible abnormal phenotypes called syndromes increases with the increase in the age of female parents due their homogametic nature. 5. Briefly describe any two of the following: (a) Significance of autopolyploid When same set of chromosomes are repeated for three or more times, it is called autopolyploid, such as AAA=Autotriploid AAAA=Autotetraploid AAAAAA=Autohexaploid

7 The autoployploids are polyploids obtained by self pollination or intra-specific hybridization followed by chromosome doubling in the hybrid either by endomitosis (natural) or by colchicines treatment. Common examples of autopolyploids are the horticultural plants such as Bananas, Grapes, Water melons, Cucurbits, Doob grass etc. Autopolyploids show increased gene dose effects leading to enhanced size of leaf, flowers, fruits, alkaloid contents etc. This phenomenon is known as Gigas effect or Gigantism. (b) Production of man made cereal Triticale, a man made cereal, is an allohexaploid produced artificially through inter-specific hybridization between tetraploid wild wheat (Triticum dicoccoides) and diploid rye plant (Secale cereale) followed by chromosome doubling by colchicines treatment in the sterile hybrids. Tetraploid wheat Rye Triticum dicoccoides X Secale cereale 2n=28 (AABB) 2n=14 (RR) ABR (sterile allotriploid 2n=21) (Colchicines treatment) 2n=42 AABBRR Fertile Allohexaplod Plant called Triticale or Wye (A man made cereal) (c) Raphanobrassica Based on the success of Triticale, a Russian scientist Karpechenko attempted to produce an inter-specific hybrid between diploid raddish (Raphanus sativus) and diploid cabbage (Brassica oleracea var. botrytis) to treat the sterile hybrid with colchicines and produce an allotetraploid variety of new plant having ability to produce raddish root and leaves of cabbage. The results of cross producing Raphano-brassica is shown below: Diploid Raddish Diploid Cabbage Raphanus sativus X Brassica oleracea 2n=18 (RR) 2n=18 (BB) RB (Sterile Hybrid 2n=18) (Colchicines treatment) 2n=36 RRBB Fertile Allotetraplod Plant called Raphanobrassica or Rabbage However, the plant could not be economically viable due to reversion in phenotypic traits.

8 6. Explain sex-influenced character giving the pattern of their inheritance in human population. Certain autosomal dominant genes in heterozygous conditions express differently in male and female humans as they are influenced by sex hormones. In homozygous conditions male and females produce similar characters but in heterozygous, the recessive gene is expressed only in males as they are influenced by male hormones. e.g. Baldness, which is a monogenic character expressed by autosomal dominant gene. If B gene gives bald phenotype and b gene produces non-bald phenotype, the probable genotypes and phenotypes in human population may be summarized as follows: Genotype Phenotype of Male Female BB Bald Bald Bb Bald Non-bald bb Non-bald Non-bald The pattern of inheritance of sex influenced characters may be described as follows. (a) If a Bald male marries with non bald females It has two possible explanation as below: Bald X Non-bald BB bb Bb (Bald if male, Non Bald, if female If four children are sons, the Bald: Non-bald ratio = 3:1 If four children are daughters, the Bald: Non-bald ratio = 1:3 7. Briefly describe the Bateson & Punett s experiment to describe two faces of linkage? The phenomenon of linkage was discovered by Bateson and Punett (1905) in sweet pea. Linkage is defined as the tendency of a set of dominant or recessive genes to remain together and pass on to next generation en bloc. Bateson & Punett (1905) suggested that linkage has two faces, as described below: Cis Trans A B A b a b a B When a dominant or recessive gene tends to remain together with another dominant or recessive gene, respectively, it is called Cis or Coupling phase. When a dominant or recessive gene tends to repel another dominant or recessive gene it is called Trans or Repulsion phase.

9 There is no difference in the genotype or phenotype of cis or trans-heterozygotes. However, the two faces of linkage in a hybrid may be easily detected by analyzing the test cross progeny as follows: Cis Trans AABB X aabb AAbb X aabb F1= AaBb AaBb Test cross experiment Cis Trans AaBb X aabb AaBb X aabb Game ab % Type Game tes ab % Type tes AB AaBb 43±7 P=50 AB AaBb 7±7 R=0 Ab Aabb 7±7 R=0 Ab Aabb 43±7 P=50 ab aabb 7±7 R=0 ab aabb 43±7 P=50 ab aabb 43±7 P=50 ab aabb 7±7 R=0 Ratio: 1:0:0:1 Ratio =0:1:1:0 P:R:R:P R:P:P:R P = Parental types R = Recombinant types The results of test cross clearly describes the difference in the Cis and trans configuration of linkage.